The modern brake master cylinder practice of placing primary and secondary pistons in tandem in a single master cylinder tends to increase the length and weight of the brake master cylinder. Increasing size and weight of accessories runs counter to the desire for smaller and lighter vehicles. In addition, in large vehicles such as trucks, the mass becomes too great to support in the usual cantilever fashion. The additional support bracket required further increases vehicle weight. One way of reducing length and consequent weight consists of removing the compensating valves, normally found in the brake master cylinder, to a location outside the cylinder. U.S. Pat. No. 3,818,706 and 3,879,947 both in the name of Robert F. Gaiser show compensating valves so located. The disclosed compensator valves are a type which are subject to blockage with dirt in the hydraulic fluid reservoir. Dirt blockage of the compensator valve is most serious when the valve is held partly open. An open compensator valve allows brake cylinder pressure to by bypassed into the brake fluid reservoir rather than going to the brake lines. Such loss of fluid pressure occasions brake failure.
The prior art fails to teach a brake master cylinder with self-cleaning compensator valve located remotely from the master cylinder.